I bought 50 swedish aspen at about 6 feet in #5 pots for resale. 34 of them have happily gone to new homes. The remaining ones have leaves that are turning brown. A given branch goes brown more or less all at once, but the affected branches are scattered all over the tree. The trees are sent into socket pots flush with the ground. Current watering regime is approximatley 1 gallon every other day, applied by drip irrigation over the span of an hour. These are the largest aspen I've got in this block. None or few of the smaller aspen are affected. (They all get the same watering.) Is this a disease, or is it a problem with my watering regime?